Stimulus-secretion coupling in the developing exocrine pancreas: secretory responsiveness to cholecystokinin

Connexin 36 is expressed in beta and connexins 26 and 32 in acinar cells at the end of the secondary transition of mouse pancreatic development and increase during fetal and perinatal life

To identify when during fetal development connexins (Cxs) 26 (Cx26) 32 (Cx32), and 36 (Cx36) begin to be expressed, as well as to characterize their spatial distribution, real time polymerase chain reaction and immunolabeling studies were performed. Total RNA from mouse pancreases at 13 and 18 days postcoitum (dpc) and 3 days postpartum (dpp) was analyzed. In addition, pancreatic sections of mouse at 13, 14, 15, 16, 18 dpc and 3 dpp and of rat at term were double labeled with either anti-insulin or anti-α-amylase and anti-Cx26 or -Cx32 or -Cx36 antibodies and studied with confocal microscopy. From day 13 dpc, Cxs 26, 32, and 36 transcripts were identified and their levels increased with age. At 13-14 dpc, Cxs 26 and 32 were localized in few acinar cells, whereas Cx36 was distributed in small beta cell clumps. From day 14 dpc onwards, the number of labeled cells and relative immunofluorescent reactivity of all three Cxs at junctional membranes of the respective cell types increased. Cxs 26 and 32 colocalized in fetal acinar cells. In rat pancreas at term, a similar connexin distribution was found. Relative Cxs levels evaluated by immunoblotting also increased (two-fold) in pancreas homogenates from day 18 dpc to 3 dpp. The early cell specific, wide distribution, and age dependent expression of Cxs 26, 32, and 36 during fetal pancreas ontogeny suggests their possible involvement in pancreas differentiation and prenatal maturation.

Novel localization of Rab3D in rat intestinal goblet cells and Brunner's gland acinar cells suggests a role in early Golgi trafficking

Rab3D is a small GTP-binding protein that associates with secretory granules of endocrine and exocrine cells. The physiological role of Rab3D remains unclear. While it has initially been implicated in the control of regulated exocytosis, recent deletion-mutation studies have suggested that Rab3D is involved in the biogenesis of secretory granules. Here, we report the unexpected finding that Rab3D also associates with early Golgi compartments in intestinal goblet cells and in Brunner's gland acinar cells. Expression of Rab3D in the intestine was demonstrated by SDS-PAGE and Western blot analysis of homogenates prepared from the rat duodenum and colon. Confocal laser scanning microscopy revealed Rab3D immunofluorescence in the Golgi area of goblet cells of the duodenum and colon and in Brunner's gland acinar cells. There was no colocalization between Rab3D and a trans-Golgi network marker, TGN-38. In contrast, Rab3D colocalized partially with a cis-Golgi marker, GM-130, and with a marker of cis-Golgi and coat protein complex I vesicles, beta-COP. Strong colocalization was observed between Rab3D and the lectins Griffonia simplicifolia agglutinin II and soybean agglutinin, which have been described as markers of the medial and cis-Golgi, respectively. Rabphilin, a putative effector of Rab3D, displayed an identical pattern of Golgi localization. Incubation of colon tissue with carbamylcholine or deoxycholate to stimulate exocytosis by goblet cells caused a partial redistribution of Rab3D to the cytoplasm and mucous granule field and a concomitant transformation of the Golgi architecture. Taken together, the present data suggest that Rab3D and rabphilin may regulate the secretory pathway at a much earlier stage than what has hitherto been assumed.

Delayed expression of large conductance K+ channels reshaping agonist-induced currents in mouse pancreatic acinar cells

Epithelial secretory cells display cell-specific mechanisms of fluid secretion and express large conductance voltage- and Ca2+-activated K+ (Maxi-K) channels that generate the membrane negativity for effective Cl- exit to the lumen. Rat and mouse pancreatic acinar cells had been thought to be peculiar in this sense because of the previously reported lack of Maxi-K channels. However, this view is not entirely correct as evidenced in the present paper. Searching for their presence in pancreatic acinar cells in mice from 5 to 84 weeks of age with patch-clamp current measurements, we demonstrated that the expression of Maxi-K channels is regulated in an age-associated manner after birth. The expression started at approximately 12 postnatal weeks and increased steadily up to 84 weeks. In support of this, RT-PCR could not detect mSlo mRNA, the Maxi-K gene, at either 7 or 8 weeks but could at 58 and 64 postnatal weeks. These results suggest that a key steering element for fluid secretion, the Maxi-K channel, is progressively re-organized in rodent pancreas. A pancreatic secretagogue, acetylcholine, evoked Maxi-K channel current overlapping to various degrees on the previously known current response. This suggests that the rise in internal Ca2+ activates Maxi-K channels which reshape the mode of secretagogue-evoked current response and contribute to Cl- driving in fluid secretion in an age-associated fashion.

Membrane targeting in secretion

Regulated secretion and exocytosis require the selective packaging of regulated secretory proteins in secretory storage organelles and the controlled docking and fusion of these organelles with the plasma membrane. Secretory granule biogenesis involves sorting of secretory proteins and membrane components both at the level of the trans-Golgi network and the immature secretory granule. Sorting is thought to be mediated by selective protein aggregation and the interaction of these proteins with specific membrane domains. There is now considerable interest in the understanding of the complex lipid-protein and protein-protein interactions at the trans-Golgi network and the granule membrane. A role for lipid microdomains and associated sorting receptors in membrane targeting and granule formation is vividly discussed for (neuro)endocrine cells. In exocrine cells, however, little has been known of granule membrane composition and membrane protein function. With the cloning and characterization of granule membrane proteins and their interactions at the inner leaflet of zymogen granules of pancreatic acinar cells, it is now possible to elucidate their function in membrane targeting and sorting of zymogens at the molecular level.

Carboxyl-methylation of Rab3D in the rat pancreatic acinar tumor cell line AR42J

Rab3D is a small GTPase implicated in regulated exocytosis, and is a marker of secretory granules in exocrine cells. We have previously shown that rab3D undergoes reversible carboxyl-methylation in adult rat pancreatic acinar cells, and that carboxyl-methylation of rab3D is developmentally regulated concomitantly with the maturation of the regulated secretory apparatus in rat pancreas. We also observed that dexamethasone treatment of the rat pancreatic acinar tumor cell line, AR42J, led to a significant increase in the size of the unmethylated pool of a rab3-like protein. The current study was designed to further characterize this rab3-like protein. Here we show that AR42J cells express rab3D, and that the protein focuses on 2D gels as two spots with pI values of 4.9 and 5.0. Treatment of AR42J cells with N-acetyl-S-geranylgeranyl-l-cysteine, an inhibitor of carboxyl-methylation, led to a decrease in the basic form of rab3D and a proportional increase in the acidic form. In contrast, N-acetyl-S-farnesyl-l-cysteine, which inhibits carboxyl-methylation of farnesylated proteins, had no effect. Lovastatin, an inhibitor of geranylgeranylation, also induced an accumulation of the acidic form of rab3D. Taken together, these data indicate that rab3D can undergo reversible carboxyl-methylation in AR42J cells by a geranylgeranyl-specific methyltransferase. The 2D gel and immunoblotting analyses indicated that dexamethasone treatment of AR42J cells led to an increase in the proportion of the unmethylated form of rab3D concurrent to inducing a regulated secretory pathway, similar to the rab3D profile change in developing rat pancreas. Our data, along with previous studies done on developing rat pancreas, indicate that the tumor cell line AR42J represents a good model system for studying the regulated secretory pathway, and that carboxyl-methylation of rab3D may play a role in the acquisition of stimulus-secretion coupling.

Ontogeny and species differences in the pancreatic expression and localization of the CCK(A) receptors

We have evaluated the presence and localization of the CCK(A) receptor in rat, mouse, pig and human fetal pancreas by Northern, Western blots and immunofluorescence techniques. In the rat, parallelism exists between development of the CCK(A) receptor mRNA and protein with maximal peaks of expression during the suckling period. In the course of pancreatitis induction, CCK(A) receptor mRNA were maximally expressed and sustained during the gland's regeneration. In the rat and mouse pancreas, the CCK(A) receptor protein is localized around the acinar cells and beta cells of the islets of Langerhans. In the adult pig and fetal human pancreas, the CCK(A) receptor proteins were detected by Western blot. By immunofluorescence, its detection was possible only in the islet of Langerhans of the pig pancreas. These new findings support the views that CCK plays important and various roles in specific physiological systems of the pancreas of different species.

Characterization of cell volume-sensitive chloride currents in freshly prepared and cultured pancreatic acinar cells from early postnatal rats

1. In freshly prepared and cultured exocrine pancreatic acinar cells from 5- to 7-day-old rats a chloride-selective membrane conductance could be activated by intracellular application of GTPgammaS (40-100 microM), by application of positive pressure (5 cmH2O) to the pipette interior or by challenging the cells with a hyposmolar bath solution. Hyperosmolar bath solutions inhibited the cell volume-sensitive chloride currents. 2. The anion permeability sequence of the cell volume-sensitive chloride conductance was I- > Cl- approximately Br- > F- > methanesulphonate- > glutamate-. I- had a higher permeability but lower conductance than Cl-. The permeability ratio for Pglutamate/PCl was 0.12. 3. The cell volume-sensitive chloride conductance showed outward rectification. Membrane depolarization to high positive voltages (>= +60 mV) caused a time-dependent decay in outward currents. 4. DIDS (4, 4'-diisothiocyanatostilbene-2,2'-disulphonic acid) and SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) reversibly inhibited the cell volume-sensitive chloride current in a voltage-dependent manner. NPPB (5-nitro-2-(3-phenylpropylamino)-benzoic acid), quinidine, quinine and tamoxifen caused voltage-independent current inhibition. 5. Combined fura-2 and whole-cell current measurements showed that activation of the cell volume-sensitive chloride current does not involve cytosolic Ca2+ signals. Furthermore, there is no evidence that Ca2+-activated chloride currents play a significant role in cultured pancreatic acinar cells from 5- to 7-day-old rats. 6. Polymerase chain reaction followed by DNA sequence analysis indicated the presence of mRNA homologous to the ClC-3 chloride channel in pancreatic tissue from 5-day-old rats.

Carboxyl methylation of rab3D is developmentally regulated in the rat pancreas: correlation with exocrine function

Several GTPases of the rab family, including rab3A, are methylesterifled on their carboxy-terminal prenylcysteine residue. The significance of this reversible posttranslational modification for the function of rab proteins is unknown, although it has been postulated that carboxyl methylation facilitates the membrane association of prenylated proteins through a hydrophobic mechanism. We here demonstrate, that pancreatic rab3D undergoes developmentally regulated carboxyl methylation concurrently with the maturation of the regulated secretory apparatus in pancreatic acinar cells: in fetal glands, which are refractive to hormone stimulation, the majority of the rab3D protein was methylated, whereas in neonatal and adult glands, which are secretory competent, only 50% was methylated. The methylated form of rab3D was also predominant in a transplantable acinar cell tumor which displays impaired secretory responsiveness and morphological characteristics reminiscent of the fetal pancreas. In addition, treatment of AR42J pancreatic acinar tumor cells with dexamethasone to induce a regulated secretory pathway, led to a significant increase in the size of the unmethylated pool of a rab3-like protein. Strikingly, membrane preparations from adult pancreata and parotid glands contained both methylated and unmethylated forms of rab3D indiscriminately. These results suggest that the acquisition of stimulus-secretion coupling by the exocrine pancreas correlates with the methylation state of rab3D, and that carboxyl methylation plays no significant role in enhancing the membrane association or determining the subcellular distribution of rab3D.

Role of laminin-1, collagen IV, and an autocrine factor(s) in regulated secretion by lacrimal acinar cells

Adhesion to novel basement membrane component BM180 in the presence of laminin-1 promotes stimulus-secretion coupling in lacrimal acinar cells [G. W. Laurie, J. D. Glass, R. A. Ogle, C. M. Stone, J. R. Sluss, and L. Chen. Am. J. Physiol. 270 (Cell Physiol. 39): C1743-C1750, 1996]. The identity of the active laminin-1 site and the possibility that other promoters of coupling are present in the acinar cell microenvironment were probed by use of different substrates, media, neutralizing antibodies and cell numbers. Regulated peroxidase secretion was unaffected by basement membrane coat concentration and was detectable at reduced levels in serum-free medium. Anti-laminin-1 antibodies, particularly against sites in the beta1 and gamma1 chains, but not alpha1 chains, partially suppressed regulated secretion, as did an anti-collagen IV antibody. Without effect were RGD peptide and antibodies against entactin, the beta1-integrin subunit, and several growth factors. Increasing cell number in serum-free medium revealed an unknown, serum-maskable, secretion-enhancing activity with a remarkable specificity for regulated secretion. Stimulus-secretion coupling, therefore, appears to be modulated by several extracellular factors whose relative contributions remain to be determined.

Molecular cloning, developmental expression and pharmacological characterization of the CCKB/gastrin receptor in the calf pancreas

We have cloned the calf predominant pancreatic cholecystokinin B (CCKB)/gastrin receptor cDNA. It encodes a 454 amino acid protein with 90% identity with the CCKB/gastrin receptor cloned in other species and tissues. However, the calf pancreatic CCKB/gastrin receptor contains a pentapeptide cassette within the third intracellular loop which is absent in the cloned human brain and stomach receptor. Quantification of the CCKB/gastrin receptor mRNA levels by reverse transcription polymerase chain reaction demonstrated the same level of transcripts at birth, +7 and +28 days. On the other hand, binding study with pancreatic membranes showing a dramatic increase (600-fold) in the number of CCKB/gastrin receptor sites between at birth and +28 days indicates that the development of the calf pancreatic CCKB/gastrin receptor occurs during the first 4 weeks of post-natal life. COS monkey cells (COS-7 cells) transiently transfected by the cloned cDNA exhibit binding of 125I-Bolton-Hunter-[Thr28,Ahx31]CCK-(25-33) and 125I-Bolton-Hunter-[Leu15]human gastrin-(2-17) to two affinity classes of sites. Kd values of the high affinity binding components indicate a 4-fold higher affinity of the receptor for sulfated gastrin than for CCK. Finally, the recombinant receptor is coupled to G proteins and [Ca2+]i mobilization, and is expressed as a glycoprotein of 82 kDa.

Lineage-specific morphogenesis in the developing pancreas: role of mesenchymal factors

Pancreatic organogenesis has been a classic example of epitheliomesenchymal interactions. The nature of this interaction, and the way in which endocrine, acinar and ductal cell lineages are generated from the embryonic foregut has not been determined. It has generally been thought that mesenchyme is necessary for all aspects of pancreatic development. In addition islets have been thought to derive, at least in part, from ducts. We microdissected 11-day embryonic mouse pancreas and developed several culture systems for assays of differentiation: (i) on transparent filters; (ii) suspended in a collagen I gel; (iii) suspended in a basement membrane rich gel; (iv) under the renal capsule of an adult mouse. Epithelia were grown either with or without mesenchyme, and then assayed histologically and immunohistochemically. Epithelium with its mesenchyme (growth systems i-iv) always grew into fully differentiated pancreas (acinar, endocrine, adn ductal elements). In the basement membrane-rich gel, epithelium without mesenchyme formed ductal structures. Under the renal capsule of the adult mouse the epithelium without mesenchyme exclusively formed clusters of mature islets. These latter results represent the first demonstration of pure islets grown from early pancreatic precursor cells. In addition, these islets seemed not to have originated from ducts. We propose that the default path for growth of embryonic pancreatic epithelium is to form islets. In the presence of basement membrane constituents, however, the pancreatic analage epithelium appears to be programmed to form ducts. Mesenchyme seems not to be required for all aspects of pancreatic development, but rather only for the formation of acinar structures. In addition, the islets seem to form from early embryonic epithelium (which only express non-acinar genes). This formation occurs without any specific embryonic signals, and without any clear duct or acinus formation.

Development and regulation of porcine pancreatic function

A surgical and experimental procedure was developed to enable the collection of pure and inactivated pancreatic juice during the growth of the pig. Studies have shown that, during the suckling period, both the basal and the secretory responses to suckling are low, if present at all. After weaning, basal levels of the total exocrine secretion, total protein, amylase, and trypsin, respectively, increase slightly, while the postprandial levels of total protein, amylase, trypsin, lipase, colipase, and carboxylester lipase, respectively, increase markedly. The pancreatic juice enzyme composition changes qualitatively and the antibacterial activity of the pancreatic juice also significantly increases. Piglet age appeared to be of minor importance, since weaning at either 4 or 6 wk of age gave the same results. Secretin and CCK administered together in supraphysiological doses only significantly affect exocrine function from 3-4 wk of age. However, CCK may also affect the exocrine pancreas indirectly via reflexes initiated intraduodenally. Milk consumption in the suckling pig leads to a postprandial increase in glucose levels but not insulin. Milk appears to be able to regulate the exocrine pancreas to produce only the amount and type of enzymes required for digestion. Thus, milk components or digestive products may affect pancreas function regulation. Studies show that enterostatin, the procolipase activation peptide, may inhibit pancreatic secretion mediated indirectly through the GI tract. Pancreastatin, an endocrine peptide, inhibits both insulin secretion and protein and trypsin secretion to pancreatic juice. In hypoinsulinemic (alloxan+streptozotocin diabetes) pigs (15-20 kg), no postprandial pancreatic juice response is seen, although CCK 33 + secretin can stimulate pancreatic secretion. Hypoinsulinemic pigs have a reduced capacity for glucose tissue utilization, suggesting that tissue metabolism and exocrine pancreas secretion are related.

Pharmacological and biochemical characterization of cholecystokinin/gastrin receptors in developing rat pancreas. Age-related expression of distinct receptor glycoforms

Cholecystokinin/gastrin receptors in the pancreas of newborn (3-day-old) rats are of type A, as in control mature rats, revealed by pharmacological analysis of specific 125I-Bolton-Hunter-reagent-labelled [Thr34,Ahx37]cholecystokinin(31-39) (Ahx, aminohexanoic acid) binding. Also, by 1 day post-partum, pancreatic cholecystokinin receptors were shown to be coupled to guanine-nucleotide-binding regulatory (G) proteins. Scatchard analysis of 125I-Bolton-Hunter-reagent-labelled [Thr34,Ahx37]cholecystokinin(31-39) binding to pancreatic membranes from rats at different times after birth showed a slight increase in the binding capacity of cholecystokinin receptors between days 3 and 14 and a sixfold increase in 21-day-old rats, with no change in receptor affinity during development. SDS/PAGE analysis of pancreatic membranes affinity labelled with the photoactivable ligand 125I-[2-(p-azidosalicylamido)-1,3'-dithiopropionate]-labelled [Thr34,Ahx37]cholecystokinin-(31-39) identified cholecystokinin receptors of 100-135 kDa in 3-day-old rats, 96-130 kDa in 7-day-old rats, 90-125 kDa in 10-day-old rats and 85-100 kDa in 14-day-old and 21-day-old rats, as found in control adult rats. Endo-beta-N-acetylglucosaminidase F treatment yielded a core protein of 42 kDa in all developmental stages. These findings are consistent with an age-related postnatal expression of distinct glycoforms of pancreatic cholecystokinin receptors. Furthermore, it was observed that the period 2-3 weeks after birth, characterized by stabilization of the mass of the cholecystokinin receptor, precedes the dramatic increase in the receptor number.

Secretory pathways in animal cells: with emphasis on pancreatic acinar cells

Studies over the past three decades have clearly established the existence of at least two distinct pathways for the intracellular transport and release of secretory proteins by animal cells. These have been identified as the regulated and constitutive pathways. Many observations have indicated that in certain cells, such as those of the exocrine pancreas and parotid glands at least, these pathways coexist in the same cells. Although the general scheme of protein transport within these pathways is well established, many fundamental aspects of intracellular transport remain to be unraveled. How are proteins transported through the endoplasmic reticulum? How are the transitional vesicles formed and what are the underlying mechanisms involved in their fusion with the cis-Golgi cisterna? Even the general mode of transfer through the Golgi stack is debated: Is there a diffusion through the stack by flow through intercisternal tubules and openings or is there a vesicle transfer system where membrane quanta hop from one cisterna to the other? What is the fate of secretory proteins in the trans-Golgi area and by what mechanisms is a fraction of newly synthesized molecules of a given secretory protein released spontaneously while the majority of such nascent molecules are diverted into a secretory granule compartment? In this review, we have examined these and other aspects of intracellular transport of secretory proteins using pancreatic acinar cells as our reference model and we present some evidence to support the existence of a paragranular pathway of secretion associated with secretory granule maturation.

Gallbladder CCK receptors: species differences in glycosylation of similar protein cores

Receptors for cholecystokinin (CCK) on gallbladder muscularis smooth muscle have different apparent sizes in man (Mr = 85,000-95,000) and cow (Mr = 70,000-85,000). In this work, these receptors were demonstrated to represent N-linked complex glycoproteins with Mr = 43,000 protein cores, based on lectin-affinity chromatography and the deglycosylation of bands affinity labeled with 125I-D-Tyr-Gly-[(Nle28,31, pNO2-Phe33)CCK-26-33] using neuraminidase, O-glycanase and endoglycosidases H and F. Similarities in the core proteins were further demonstrated by Staphylococcus aureus V8 protease peptide mapping, in which both proteins yielded similar fragment patterns. Thus, gallbladder CCK receptors present in man and cow are both N-linked complex glycoproteins, with different carbohydrate domains and similar protein cores.

Specific regulation of the gene expression of some pancreatic enzymes during postnatal development and weaning in the calf

The construction of cDNA library from calf pancreas allowed us to examine the mRNA levels of four pancreatic hydrolases (chymotrypsin, lipase, trypsin and amylase) during postnatal development in preruminant and ruminant animals. The lack of parallel variations in the levels of the enzyme specific activities suggested that protein synthesis was not coordinately regulated. In preruminant calves, the change in chymotrypsin and lipase mRNA concentrations (0-28 day period) and in trypsin mRNA concentrations (0-119 day period) was opposite to that in the corresponding specific activities. In contrast, both the activity and mRNA profiles of amylase during the latter period, on the one hand, and those of chymotrypsin and lipase during the 28-119 day period, on the other hand, were comparable. However, the extent to which the specific activity and mRNA concentration of each enzyme were increased did not necessarily coincide. The observed changes in mRNA levels probably resulted from some transcriptional control of the gene expression and/or variation in mRNA stability. Moreover, a translational regulation of the messengers could explain the existence of non-parallel mRNA and specific activity profiles. In sharp contrast with the multiple control of protein synthesis during postnatal development in preruminant calves, weaning was found to induce the same increase in enzyme activity and corresponding mRNA for each of the four pancreatic enzymes, suggesting that pretranslational modulation of gene expression was mainly, if not exclusively, concerned.

The asynchronous transport of secretory proteins in the exocrine pancreas. Compatibility with the hypothesis of a paragranular pathway?

An asynchronous transport of individual secretory proteins has been recently described in the pancreas. This asynchrony was observed in both unstimulated and stimulated conditions. It has also been proposed that unstimulated and stimulated secretions correspond to distinct secretory processes. Indeed according to that hypothesis, under resting conditions, a small fraction of the newly synthesized secretory proteins are channeled into a paragranular (vesicle) pathway while the residual proteins are packaged in the zymogen granules. These zymogen granules eventually move to the cell surface where their content is extruded by exocytosis. Under stimulated conditions the latter process is accelerated. Since the same type of asynchrony is observed under resting and stimulated conditions in the pancreatic juice, one can wonder if the hypothesis of a paragranular pathway is compatible with the observed asynchrony. In this review, an explanation is presented to account for the facts that following pulse and chase labelling, two waves of labelled proteins are released under resting secretions and secondly that asynchrony is maintained in both resting and stimulated conditions.

Immature stimulus-secretion coupling in the developing exocrine pancreas and ontogenic changes of protein kinase C

Previous observations have shown unresponsiveness of pancreatic acini to cholecystokinin C-terminal octapeptide (CCK-8) and cholinergic agents in newborn rats. In this study, the possibility that a lack of protein kinase C may be one factor limiting the responsiveness of the acini was examined. In the term fetus and in newborns cytosolic protein kinase C activity was low. Shortly after birth, the activity increased rapidly and by 2 days of age reached adult levels which were 5-fold higher than that in the newborn. No differences in subcellular distribution of protein kinase C activity between the particulate and the cytosol fractions were found at any age studied. Developmental profiles of phorbol dibutyrate binding, an alternative method for measuring protein kinase C, were similar to those of protein kinase C activity measurements. Using stimulation of amylase secretion as an index of responsiveness, dispersed pancreatic acini of newborn rats were found to be unresponsive to TPA (a potent activator of protein kinase C) and CCK-8, but were responsive to dibutyryl cAMP and calcium ionophore A23187 (agents not dependent on protein kinase C activity). These results suggest that the low levels of pancreatic protein kinase C in newborn rats are at least in part responsible for the unresponsiveness of pancreatic acini to 12-O-tetradecanoylphorbol 13-acetate and CCK-8.

Tubulin tyrosination in Crithidia: modifying enzymes and modification states of tubulin

An enzyme that adds C-terminal tyrosine to tubulin has been identified in Crithidia fasciculata. It tyrosinates Crithidia, but not brain, tubulin and is specific for the alpha chain. Crithidia cells could not be shown to fix tyrosine in the absence of protein synthesis, which is consistent with the pattern of distribution of C-terminal tyrosine in tubulin from different subcellular compartments of this protozoan. Terminal tyrosine was present in about 5% of flagellar alpha chain from cells in stationary phase and 20% from cells from midlog phase; none was detected in tubulin from cytosol or the subpellicular corset. In contrast to mammalian cells, in which a higher state of tyrosinolation characterizes recently assembled or unstable microtubules, terminal tyrosine was present only in the most stable polymer, the flagellar doublet microtubules.

Postnatal development of biliary and pancreatic exocrine secretion in piglets

1. The secretory responses of bile and exocrine pancreas were studied in various aged piglets. 2. At 3 days old the bile and exocrine pancreas could be reacted by various stimulations. The response by secretin was the same as that in the 28 day old. 3. Protein concentration in pancreatic juice by CCK-8 increased steeply after 6 days old, but the ratio of amylase to protein rose abruptly at 28 days old. 4. These findings indicate that (1) the secretory capacity of bile and pancreatic juice developed predominantly at an early period of postnatal life; (2) the formation of bile acids and pancreatic digestive enzymes developed gradually during the suckling period.

Development of secretagogue responsiveness in the pancreas

Epithelial cells are bound by a plasma membrane that is divided into apical and basolateral domains. In turn, the species of membrane proteins within these polarized plasmalemmal domains define the structural and functional specializations of the epithelium. Currently, research into the mechanisms whereby epithelial cell polarity is generated is a major thrust in cell biology. The topic discussed here will focus on recent studies carried out by us on the genesis of regulated secretion in the developing pancreatic acinar cell--that is, the secretory pathway responsive to second-messenger elevation after hormonal stimulation. In the developing rat pancreas, acinar cells complete cytodifferentiation 1 day before birth yet do not discharge secretory proteins from zymogen granules in response to the secretagogue cholecystokinin (CCK). The cause of this refractoriness to CCK does not lie at the receptor level, since the number, affinity, and plasmalemmal location of the receptor and its ability to generate second messengers after interaction with the ligand has already appeared. However, 1 day after birth, the acinar cell is able to discharge secretory proteins by exocytosis through the regulated secretory pathway in response to CCK. The acquisition of distal stimulus-secretion coupling events in the perinatal period has been found to correlate with developmental changes in Ca2+/calmodulin-dependent protein kinase II and its phosphorylated substrates. The relevance of the studies reported here to the complex and coexistent regulated and constitutive secretory pathways in the pancreatic acinar cell will be discussed.

Biochemical characterization of the pancreatic cholecystokinin receptor: a possible marker of cell differentiation and development

The biochemical expression of the cholecystokinin (CCK) receptor on the surface of the pancreatic acinar cell is a potential marker of the state of differentiation of that cell. In this report we review the basis for and the results of the use of affinity labeling techniques for the biochemical characterization of this macromolecular receptor assembly on the adult rat pancreatic acinar cell. A series of specially designed molecular probes are used to define the subunit structure of this receptor, based on the relationships between the sites of covalent attachment of these probes and their receptor-binding domains. We suggest that the receptor-binding domain resides on a Mr = 85,000-95,000 subunit, whereas a distinct Mr = 80,000 also exists as part of this complex.

Pathways of secretion in the exocrine pancreas: the status of resting secretion

In the last decade the concept of two distinct pathways of secretion in the exocrine pancreas has slowly emerged. According to this concept, one pathway is involved in stimulated (regulated) conditions and another under resting (constitutive) conditions. This hypothesis was elaborated at first from the comparison of the specific radioactivities of secretory proteins released by the gland under resting and stimulated conditions. Analysis of the protein composition of the juice released under these two physiological conditions further supported that hypothesis. More recent studies compared the kinetic of accumulation of newly synthesized proteins in zymogen granule and their release in the gland lumen. The latter results are in agreement with a model in which secretory proteins are channelled in two separate pathways, one regulated, and one constitutive. Essentially, the constitutive pathway would correspond to a paragranular route in which the proteins would be immediately secreted instead of being stored in zymogen granules. In addition, some of the proteins released in the juice under "resting" conditions are associated to microvesicles. The term "microvesicular secretion" is used to designate that type of secretion.